Method of surge protection for a dynamic compressor using a surge parameter

ABSTRACT

A method of surge protection for a dynamic compressor that has a corresponding compressor map. A control system continually calculates an equivalent polytropic head parameter in order to define a surge limit line. The system then calculates a control parameter and determines the distance the control parameter to the surge limit line wherein the control parameter is dynamic to changes in compressor load and invariant to changes in suction conditions and gas compressibility. As a result of the distance of the control parameter to the surge limit line, the surge valve of a dynamic compressor is actuated to prevent surge.

BACKGROUND OF THE INVENTION

The present invention is directed toward a dynamic compressor. Morespecifically, the present invention is directed toward a method of surgeprotection utilizing an equivalent map surge parameter.

A typical dynamic compressor has a gas inlet and a gas outlet whereinthe compressor is driven by a compressor driver so that the gas, whileflowing through the compressor, is compressed. A problem associated withdynamic compressors is the amount of gas that passes through thecompressor. Specifically, if an insufficient amount of gas flows throughthe compressor, a surge occurs within the system causing damage to thecompressor. Because of the high price of compressors great care must betaken to ensure that compressors are not damaged.

To minimize damage to compressors as a result of lack of gas flow at aninlet an anti-surge or recycling valve is utilized by dynamiccompressors to take gas from the outlet of the compressor and recycle itback to the inlet of the compressor to ensure that there is alwayssufficient gas flowing though the compressor to prevent surges fromoccurring.

As a result of the need to protect against surge, control systems havebeen provided to control the operation of the anti-surge valve.Compressor surge control systems (also known as anti-surge controllers)use a PID controller for regulating the anti-surge valve when flow ratedecreases below a predefined point.

Control systems in the art monitor the dynamic compressor system anddetermine a corresponding compressor map as can be seen in U.S. Pat. No.4,156,578 to Agar and U.S. Pat. No. 4,949,276 to Staroselsky. In bothreferences, a function of volumetric flow at the inlet of the compressoris charted against the polytropic head of the compressor to determine asurge line or surge limit line. The surge limit line represents the lineon the graph that once passed (a point immediately to the left of thesurge limit line) surging of a compressor can occur. Thus, to preventsurging a safety margin is determined and a surge control line isplotted to the right of the surge limit line. The control system thencontinuously calculates a control parameter that measures a distance tothe surge limit line. If the control parameter reaches or is to the leftof the surge control line, the controller actuates the anti-surge valveto increase gas flow through the compressor to prevent the controlparameter from reaching the surge limit line and causing a surge withinthe compressor.

Problems in systems such as that taught by Agar and Staroselsky existbecause measuring the volumetric flow and the polytropic head inpractice is not practical. There are problems associated with molecularweight and gas density determinations causing these measurements to beinadequate for real time surge protection. Hence, controllers in theindustry employ either fan law method or use similitude theory to derivesurge control parameters that in theory are invariant to changes insuction conditions or gas composition. However, existing methods forinvariant parameter calculations do not completely account forvariability in gas compressibility or gas specific heat ratio. As aresult, variations in gas composition tend to make the surge parameterand surge limit line move resulting in operating problems. In addition,existing methods for the distance to surge calculation method isdynamically insensitive or sluggish especially as the compressor loadincreases. Thus, for the existing methods, changes in distance to thesurge line is smaller for a given change in compressor load.

Therefore, a principal objective of the present invention is to providea method of surge protection for a dynamic compressor that preventsdamage to the dynamic compressor.

Yet another objective of the present invention is to provide a method ofsurge protection for a dynamic compressor that accounts for multiplevariables in determining a control parameter.

These and other objectives, features, or advantages of the presentinvention will become apparent from the specification and claims.

BRIEF SUMMARY OF THE INVENTION

A method of surge protection for a dynamic compressor having acorresponding performance map. The method includes continuallycalculating an equivalent polytropic head parameter and an equivalentflow parameter. Next, the method involves defining a surge limit line onthe compressor map as a function of the equivalent polytropic headparameter. Then a control system continually calculates a distance acontrol parameter is from the surge limit line wherein the controlparameter is dynamic to changes in compressor load. Based on thedistance an anti-surge valve is adjusted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a dynamic compressor;

FIG. 2 is a block diagram of a control system of a dynamic compressor;

FIG. 3 is a schematic diagram of a surge control system of a dynamiccompressor; and

FIG. 4 is a graph having equivalent flow squared versus equivalentpolytropic head.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a dynamic compressor 10 that includes a compressor 12 thatis driven by a compressor driver 14. The compressor driver is of anytype including a motor, gas turbine, steam turbine, expander or thelike. The compressor 12 has a gas inlet 16 and a gas outlet 18 whereingas flows through the compressor 12 to be compressed. An anti-surge orrecycle valve 20 is fluidly connected between the gas inlet 16 and gasoutlet 18 so that when the anti-surge valve 20 opens a fluid flow pathexists to convey gas from the gas outlet 18 to the gas inlet 16. Aplurality of sensors 22 including pressure sensors, temperature sensors,flow measurement sensors and the like are placed throughout the dynamiccompressor 10 in order to determine processed conditions for thecomponents of the dynamic compressor including the compressor 12, thedriver 14, the gas inlet 16, and gas outlet 18 and the anti-surge valve20. The plurality of sensors 22 are electrically connected to thecontrol system 24 where the control system 24 is in communication withall of the components of the dynamic compressor and controls the openingof the anti-surge valve 20.

FIG. 2 shows a control system 24 used for a dynamic compressor 10.Specifically, a controller 26 such as a Network Master controller isutilized in combination with a load controller 28 that monitors theinlet 16 of the dynamic compressor 10 and a surge controller 30 thatmonitors and operates the anti-surge valve 20. The arrangement and setup of the control system 24 shown in FIG. 2 is merely an example of acontrol system 24 for a dynamic compressor 10 and is shown as exemplary.Specifically, other control arrangements that utilize Mastercontrollers, load controllers, and surge controllers in series cascadeand parallel cascade can be used without falling outside the scope ofthe present invention.

FIG. 3 is yet another exemplary embodiment of the control system 24 usedwith another dynamic compressor 10. In this embodiment the station andload controllers are not used and instead the control system 24 takesreadings straight from the plurality of sensors 22 to determine an inputto the surge valve 20.

The control system 24 of the present invention, like prior art controlsystems, determines a compressor map that corresponds to the dynamiccompressor 10 as best shown in FIG. 4. In the present invention thecompressor map 32 presents a horizontal axis 34 that measures the squareof an equivalent flow q² _(eq) and has a vertical axis 36 that presentsan equivalent polytropic head. On the map 32 is a surge limit line 38that is calculated by the control system 24 wherein at points to theleft of the surge limit line 38, surge within the dynamic compressor 10typically occurs. Spaced at a predetermined distance that is considereda safety margin 40 is a surge control line 42 wherein when a controlparameter reaches a point either on or to the left of the surge controlline 42 the control system 24 actuates the anti-surge valve 20 toprovide flow through the anti-surge valve 20. Additionally seen on themap 32 are operating control lines 44 that represent additional controllines that are used if the safety margin 40 is desired to be increasedto protect against a surge within a compressor.

The compressor map 32 of the present invention is an equivalentcompressor map wherein instead of attempting to measure inlet volumetricflow and apply it against polytropic head the present inventioncalculates an equivalent flow parameter q² _(eq) and an equivalentpolytropic head parameter h_(eq) Specifically:

$q_{eq}^{2} = \frac{Q^{2}}{V_{c}^{2}}$$q_{eq}^{2} = \frac{Q^{2}}{({nZRT})}$

and:

$h_{eq} = {\frac{H_{p}}{V_{c}^{2}} = \frac{H_{p}}{({nZRT})}}$

wherein:

$V_{c}^{2} = {{nZRT} = \frac{P}{\rho}}$

Where:

Q=volumetric flow

V_(c)=sonic velocity of gas at flow conditions

H_(p)=polytropic head

n=polytropic exponent

$R = \frac{Ro}{MW}$

where Ro is the universal gas constant and MW is molecular weight

Z=compressibility of gas at flow conditions

T=temperature of gas at flow conditions

Thus, because, for a polytropic compression process:

P_(v) ^(n)=constant and

$\frac{P}{\rho^{n}} = {constant}$

Where:

P=pressure at flow conditions

ρ=gas density at flow conditions

v=specific gas density at flow condition

Therefore:

$q_{eq}^{2} = {{c_{1}\frac{\Delta \; {Po}}{P}*\frac{1}{n}} = \frac{q_{r}^{2}}{n}}$$h_{eq} = {{c_{2}\frac{\left\lbrack {{R_{c}\frac{n - 1}{n}} - 1} \right\rbrack}{\frac{n - 1}{n}}*\frac{1}{n}} = \frac{h_{r}}{n}}$

Where:

c₁ and c₂ are constant

R_(c)=pressure ratio across the compressor

ΔPo=differential pressure across flow measuring device

so

$h_{r} = {c_{2}\frac{\left\lbrack {{R_{c}\frac{n - 1}{n}} - 1} \right\rbrack}{\frac{n - 1}{n}}}$

and

$q_{r}^{2} = {c_{1}\frac{\Delta \; {Po}}{P}}$

Thus, if variations in n are neglected then invariant surge limitparameters are q² _(r) and h_(r),Therefore, a surge parameter is represented by:

P:q ² _(eq)|surge=f(h _(eq))

and surge limit line=f(h_(eq))

In addition to the above a control parameter (R) is defined as theprocess variable for surge controller 24 and shown as:

$\begin{matrix}{R = {\frac{q_{eq}^{2}{op}}{q_{eq}^{2}{surge}} - {SM}}} \\{= {\frac{q_{eq}^{2}{op}}{f\left( h_{eq} \right)} - {SM}}}\end{matrix}$

Where:

-   -   q² _(eq)|op=the equivalent volumetric flow parameter at an        operating condition    -   q² _(eq)|surge=the equivalent volumetric flow parameter at a        surge condition    -   f(h_(eq))=a function of the equivalent polytropic head    -   SM=surge margin set point        so that when R≧1.0 anti-surge control valve 20 is closed and        when R<1.0 the anti-surge recycle valve 20 is open.

In addition, the control system 24 determines the distance to the surgecontrol line δ:

δ=[((q ² _(eq) |op)/f(h _(eq)))−SM]−1=(R−1)

so when δ is ≧0 the valve is closed and when δ is <0 the valve is open.Thus the surge controller acts on δ to actuate the surge valve andprevent surge.

In operation, as the dynamic compressor 10 is operating the controlsystem 24 continually monitors the dynamic compressor 10. The controlsystem 24 continually calculates an equivalent polytropic head parameterand an equivalent flow parameter in the manners discussed above. A surgelimit line 38 is defined on the compressor map 32 as a function of theequivalent polytropic head parameter. The control system 24 continuallycalculates a distance 5 that a control parameter R is from the surgelimit line 38 wherein the control parameter is dynamic to changes in thecompressor load. Then, as a result of the distance the control parameteris from the surge limit line the control system 24 actuates theanti-surge valve 20 accordingly.

Thus, provided is a dynamic compressor control system that utilizes anequivalent compressor map 32 to improve upon the state of the art. Theequivalent compressor map 32 bases a surge parameter on the polytropiccompression process equation and modeling of the dynamic compressor 10based on flow, pressure, speed (or inlet guide vane), compressibilityand temperatures of the dynamic compressor 10. The equivalent polytropichead parameter and equivalent flow parameter are based on the dynamicsimilitude theory, a mach number determination using sonic velocity ofgas at flowing conditions and gas compressibility.

When determining control parameter (R), the parameter is dynamic tochanges in compressor load, both in the increasing and decreasingdirection. Therefore, presented is a control parameter that has highdynamic sensitivity along with invariance of the surge equivalentparameter due to changes in suction pressure, temperature, gascomposition, rotation speed or inlet guide vane geometry. Thus, at thevery least all of the stated objectives have been met.

It will be appreciated by those skilled in the art that other variousmodifications could be made to the device without departing from thespirit and scope of this invention. All such modifications and changesfall within the scope of the claims and are intended to be coveredthereby.

1. A method of surge protection for a dynamic compressor having acorresponding compressor map steps comprising: continually calculatingan equivalent parameter with a controller; defining a surge limit lineon the compressor map as a function of the equivalent parameter;continually calculating a distance a control parameter is from the surgelimit line, wherein the control parameter is dynamic to changes incompressor load; and actuating an surge valve based on the distance. 2.The method of claim 1 wherein the equivalent parameter is an equivalentpolytropic head parameter.
 3. The method of claim 2 wherein theequivalent polytropic head is a function of $\frac{Hp}{V_{c}^{2}}$ whereHp is polytropic head and V_(c) ² is equal to NZRT where N is apolytropic exponent, R is equal to $\frac{Ro}{MW}$ where Ro is a gasconstant and MW is molecular weight and T is the temperature of gas atflow conditions at the compressor.
 4. The method of claim 2 wherein thesurge limit line is defined based on the polytropic compression processequation, Pv^(n)=constant.
 5. The method of claim 2 wherein the surgelimit line is defined based on modeling of the compressor based on flow,pressure, speed, compressibility and temperature.
 6. The method of claim2 wherein the equivalent polytropic head parameter is based onsimilitude theory.
 7. The method of claim 2 wherein the equivalentpolytropic head parameter is based on polytropic head correction usingsonic velocity of gas at flowing condition of the compressor.
 8. Themethod of claim 2 wherein the equivalent polytropic head parameter isbased on gas compressibility.
 9. The method of claim 2 wherein theequivalent polytropic head parameter is based on n=constant.
 10. Themethod of claim 1 wherein the control parameter is determined based onan equivalent flow parameter.
 11. The method of claim 10 wherein theequivalent flow parameter is a function of q² _(eq) wherein q² equals$\frac{Q^{2}}{V_{c}^{2}}$ where Q is volumetric flow and V_(c) is thesonic velocity of gas at flowing conditions.
 12. The method of claim 10wherein the equivalent flow parameter is based on similitude theory. 13.The method of claim 10 wherein the equivalent flow parameter is based onmach number determination using sonic velocity of gas at flowingcondition of the compressor.
 14. The method of claim 10 wherein theequivalent flow parameter is based on gas compressibility.
 15. Themethod of claim 10 wherein the equivalent flow parameter is based onn=constant.
 16. The method of claim 1 wherein the control parameter isdetermined based on R=(q² _(eq)|op)/(q² _(eq)|surge) where q²_(eq)|op=an equivalent volumetric flow parameter at an operatingcondition and q² _(eq)|surge=an equivalent volumetric flow parameter ata surge condition.
 17. The method of claim 1 wherein the distance acontrol parameter is from the surge limit line is determined based onthe control parameter and is a function of [((q² _(eq)|op)/f(h_(eq)))−1]where q² _(eq)|op is an equivalent flow parameter at an operatingcondition and f(h_(eq)) is a function of the equivalent polytropic headparameter.
 18. The method of claim 17 wherein when the distance acontrol parameter is from the surge limit line is greater than 0 thenthe compressor is operating to the right of the surge limit line andwhen the distance a control parameter is from the surge limit line isless than or equal to 0 then the compressor is operating to the left ofthe surge limit line.
 19. The method of claim 1 further comprising thestep of continually calculating a distance the control parameter is froma surge control line.
 20. The method of claim 19 where the surge controlline is determined based on the control parameter and is a function of[((q² _(eq)|op)/f(h_(eq))−SM]−1 where q² _(eq)|op is the equivalent flowparameter at an operating condition, f(h_(eq)) is a function of theequivalent polytropic head parameter defining the surge limit line andSM is the safety margin.
 21. The method of claim 20 wherein when thedistance a control parameter is from the surge control line is equal toor greater than 0 the surge valve is closed and when the distance thecontrol parameter from the surge control line is less than 0 the surgevalve is opened.